Digital systems commonly utilize disks as information storage devices. In order to read information from the disk or write information onto the disk, it is necessary that a head or transducer be properly positioned relative to the disk surface. This can be accomplished using a controlled linear motor which operably drives a carriage assembly to which the head or heads are joined. It is important, in reaching or capturing a target position on the disk surface, to precisely control the velocity of the head so that the head reaches the target area in a minimum amount of time. In this regard, it is desirable to avoid undershooting or, conversely, overshooting of the target position since additional and unproductive time is expended in correcting for such errors in order to properly position the head for the subsequent read or write operation.
Is known in prior art systems to continuously provide servo information for indicating to the processing system where the head is positioned relative to a disk surface. This is accomplished by means of a disk surface which is totally dedicated to stored servo information, i.e., there is no data or other non-position information on the disk surface. A corresponding disk follows or corresponds to the disk which carries the position information so that the disk drive of the prior art system utilizes the servo information from one disk to read from or write on the corresponding disk.
The concept of dedicating an entire disk surface for storing servo information works satisfactorily for relatively large computing systems utilizing large disk storage media. However, in the case of lower capacity processing systems using relatively small disk surfaces, such as about eight inches in diameter, it is not feasible to dedicate entire disk surfaces to servo information since disk storage space is at a premium and it is desirable to use as much of the disk surface as is reasonably possible for user information or data. Since position information is still necessary, regardless of disk size, for use in determining where the head is positioned relative to the disk, such information must still be stored and be accessible by the head for inputting to a control means such as a microprocessor. This is accomplished by means of an embedded servo system having disk servo sectors or slices which are dedicated to storing servo information (servo fields). More specifically, the disk surface area includes spaced servo fields, which are located between large disk surface sectors on which information or data is stored. Each of the spaced servo fields includes a disk track address for every track on the disk. This information indicates to the control means the general position of the head during its movement relative to the disk surface. The track address just accessed by the head, together with the known targeted track address from which the data is to be read or on which data is to be stored, could be used to determine a coarse distance remaining to the target position and the desired speed necessary to properly reach the target. In determining the position of the head while in a servo field, a problem arises when at that time the head is located between track centers. In this position, the head is located at some track address plus or minus the distance from the track center. In such a case, this relatively short or fine distance from one of two adjacent track centers must be taken into account, as well as the address of one of the two adjacent tracks, in order to determine the exact head position. To achieve this objective the servo fields on the disk have magnetic properties so that a Normal signal and a Quadrature signal can be generated. Each of these two signals includes pulses, the magnitudes of which relate to the distance the head is away from a track center. The Normal signal provides position information concerning the distance the head is located from a track center, while the Quadrature signal provides position information concerning the distance that the head is located relative to a distance one-half track away from the track center. This pulse information is used by the hardware and program instructions of the present invention to precisely determine the head position. From the exact head position and the known distance remaining to the target position, the current to a linear motor can be regulated to control the velocity of the head as it moves towards the target location.